Most high-efficiency solar cells are fabricated from monocrystalline Czochralski silicon (Cz-Si) wafers because the material quality is higher than multicrystalline silicon (mc-Si) wafers. However, the material study presented in this paper reveals strong variations in the material quality of commercially available Cz-Si wafers, leading to a loss in solar cell efficiency of 4% absolute. The reason for this is the presence of defects, which appear as dark rings in photoluminescence (PL) images of the finished solar cells. It is shown that these efficiency-limiting defects originate from oxygen precipitation during emitter diffusion. It is demonstrated that an incoming inspection in the as-cut state is difficult, as strong ring structures in as-cut wafers turn out to originate most often from thermal donors. These are dissolved during high-temperature treatments and are therefore harmless, whereas moderate ring structures in the as-cut state may become severe. That is why critical wafers can be identified and sorted out reliably only after emitter diffusion, by using QSSPC-based lifetime measurements or PL imaging. The two-year statistics gathered from the research line at Fraunhofer ISE on the occurrence of ring defects in Cz-Si wafers indicate that ring defects are highly relevant in terms of material yield.
Advances in nanofabrication for enhancing the efficiency of optical devices, such as solar cells and photo-detectors, via nanostructuring have attracted a great deal of interest. A photoconversion strategy employing nanorods (NRs) has emerged as a powerful way of overcoming the limitations of planar wafer-based or thin-film solar cells. But there is also a broad spectrum of challenges to be tackled when it comes to putting into practice cost-effective NR solar cell concepts. ROD-SOL is a 10-partner, ‘nanotechnology for energy’ project with end-users, equipment manufacturers and institutes from six countries forming the consortium. The aim of the project is to provide the photovoltaic market with a highly efficient (> 10%), potentially low-cost, thin-film solar cell concept on glass, based on silicon nanorods. This paper presents the project’s achievements and discusses what the future might hold for nanotech-based solar energy production.
Today, crystalline-Si photovoltaics (PV) dominate the market, accounting for more than 85% of market share in 2010. A large scientific community made up of academic as well as industrial stakeholders strives to find solutions to improve device efficiencies and to drive down costs. One of the important cost elements of a module is the c-Si wafer itself. This paper discusses the fabrication of a carpet of c-Si foils on glass, either by layer transfer of an epitaxially-grown layer or by bonding of a very thin wafer, and processing this c-Si thin-foil device into a photovoltaic module. This could constitute an advantageous meet-in-the-middle strategy that benefits not only from c-Si material quality but also from thin-film processing developments.
The improved performance and reduced manufacturing costs of photovoltaic (PV) modules that have been achieved in recent years have positioned this technology as an economically attractive renewable electric energy source. In order to verify that this also has a positive impact on energy payback time (EPBT) and carbon footprint, the Energy Research Centre of the Netherlands (ECN) has conducted a life cycle analysis (LCA) for REC Peak Energy-series PV modules produced by Renewable Energy Corporation (REC). The LCA study was based on a full set of actual production data obtained for the first quarter of 2011 from REC’s manufacturing sites. Because REC is an integrated manufacturer, the LCA study includes internal data for the production steps from polysilicon production to module assembly, as well as for all materials and transportation associated with production. ECN used generic figures for installation, operations and recycling together with the REC data to assess the environmental impact indicators. For polysilicon produced in the USA, and for wafers, cells and modules produced in Singapore, an EPBT of 1.2 years was achieved, with a corresponding carbon footprint of 21g CO2-eq/kWh for PV systems located in southern Europe (1700kWh/m2year irradiation). For modules with wafers and cells produced in Norway, the corresponding values were 1.1 years and 18g CO2-eq/kWh. A key contributor in achieving these values is REC’s highly efficient fluidized bed reactor (FBR) process for the production of polysilicon.
This paper presents a novel glue-membrane integrated backsheet specifically for PV modules, which has been designed and fabricated by utilizing a flow-tangent cast roll-to-roll coating process combined with a plasma technique. Polyethylene terephthalate (PET) is adopted as a substrate and is surface activated and etched by atmospheric plasma. Then a special coating formulation containing reactive fluoropolymers is applied to both sides of the PET, followed by thermal curing, resulting in a glue-membrane integrated coating layer with a polyurethane structure. Finally, a monolayer of silane molecules is grafted onto the surface via plasma-enhanced deposition to provide the surface medium with surface energy, rendering excellent long-term adhesion to ethylene vinyl acetate (EVA). Scanning electron microscope (SEM) images have revealed that plasma etching and activation significantly improves compatibility between the PET and the coating layer, resulting in a tight and strong integration between the two. It has also been confirmed by SEM that the obtained novel backsheet integrates the glue layer and the membrane layer perfectly. There is no clear boundary between the two layers, distinguishing the novel backsheet from the conventional layer-by-layer laminated backsheet. The unique glue-membrane integrated structure has already been demonstrated by many practical applications under harsh environmental conditions to have significant advantages over other backsheets regarding delamination, blistering and discoloration. Furthermore, the novel backsheets showed excellent barrier properties, weatherability (85°C, 85% RH, 1000h), mechanical properties and electrical isolation properties. Because it is a promising photovoltaic material, the novel backsheet has already been widely used in China for PV module encapsulation and has obtained extensive praise from customers.
In trying to introduce its relatively new technology to traditional utility customers, the photovoltaic industry often finds itself in the awkward position of trying to sell a product to a customer who may not want to buy. The up-front capital costs of new solar plants (that deliver power only intermittently) can be less than appealing. Large-scale grid integration will therefore be accelerated by PV technologies that best fit the profile of traditional power sources. In addition to low cost, this includes high capacity factors and the ability to better match demand during daylight hours.
Concentrator photovoltaic (CPV) power plants are now being integrated into the grid at megawatt scales. By performing light collection using acrylic, silicone, or glass optics instead of semiconductors, the material cost balance of PV is fundamentally shifted. The world’s most efficient solar cells can then be employed, and maintaining tracking of the sun becomes economically favorable across vast sunny locales worldwide. With AC system efficiencies in excess of 25%, the resulting CPV power plants produce high energy yields throughout the year and deliver the high capacity factors demanded by utility customers. Since semiconductors are a minority component cost, manufacturing capital costs are lower than for any other PV technology, allowing for rapid scale-up and field deployment. This article will describe the state of the art of CPV technology, field performance results, and the outlook for near-term deployments.
The PV industry is undergoing dramatic changes. Like a carnival ride gone dreadfully wrong, exhilaration has been supplanted by dread; joy has been replaced by fear. Just look around you – provided you are able to turn your head to defy the g-forces acting upon you as we bank and turn wildly along. You will see PV companies closing their doors for good. You will see extraordinarily talented people throughout the supply chain, shifting positions everywhere and looking for safe-haven jobs. And you will also see once-leading PV companies burning cash and losing their status as ‘bankable’. Everywhere we turn, we see companies in the supply chain shuttering production as if to balance markets.
The Unite d States and China are embroiled in what the New York Times has dubbed “the most politically charged trade case in many years”. Seven US-based crystalline silicon solar cell manufacturers have formed. The Coalition for American Solar Manufacturing (CASM), led by SolarWorld and First Solar (the identities of the five remaining members are yet to be disclosed). The CASM has accused Chinese manufacturers of violating global trade laws with its “anti-competitive trade aggression”.
This paper presents a new differential scanning calorimetry (DSC) method that allows the determination of the degree, or level, of crosslinking of ethylene-vinyl acetate (EVA) copolymers, including EVA films used as encapsulants for photovoltaic (PV) applications. This method can also determine additional characteristics of EVA, such as its weight per cent (wt %) vinyl acetate (VA) content and its fluidity. The paper describes the procedure and its application to EVA film samples laminated at 145°C, for different lengths of time in an industrial-type laminator for PV modules, as well as to EVA uncrosslinked samples of different composition and fluidity. The scope of the method compared to other characterization methods for the degree of crosslinking of EVA is discussed. An experimental
comparison is also made to rheological and gel content methods.
This paper describes the technical concepts and current status of back-contact module technology. A back-contact module has the advantage of a higher conversion efficiency because of less shading of the front of the cell, fewer inactive areas in the module and lower series resistance in the interconnection. Aesthetically, back-contact modules are more attractive than standard modules. Furthermore, module manufacturing is gentler due to there being less cell handling during the process. The two main technical concepts related to back-contact modules – interconnector technology and printed circuit backsheet technology – are discussed in this paper. An overview is given of the production status of current back-contact module manufacturers to also show the significant potential of this technology in economic terms.
